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Capabilities Advertisement with BGP-4

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 5492.
Authors Ravi Chandra , John Scudder
Last updated 2020-07-29 (Latest revision 2009-01-07)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Draft Standard
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 5492 (Draft Standard)
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Consensus boilerplate Unknown
Telechat date (None)
Responsible AD David Ward
Send notices to (None)
Internet Engineering Task Force                               J. Scudder
Internet-Draft                                          Juniper Networks
Obsoletes: 3392 (if approved)                                 R. Chandra
Intended status: Standards Track                           Sonoa Systems
Expires: July 11, 2009                                   January 7, 2009

                 Capabilities Advertisement with BGP-4

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 11, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
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   This document defines an Optional Parameter, called Capabilities,

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   that is expected to facilitate the introduction of new capabilities
   in the Border Gateway Protocol (BGP) by providing graceful capability
   advertisement without requiring that BGP peering be terminated.  This
   document obsoletes RFC 3392.

1.  Introduction

   The base BGP-4 specification [RFC4271] requires that when a BGP
   speaker receives an OPEN message with one or more unrecognized
   Optional Parameters, the speaker must terminate the BGP peering.
   This complicates the introduction of new capabilities in BGP.

   This specification defines an Optional Parameter and processing rules
   that allow BGP speakers to communicate capabilities in an OPEN
   message.  A pair of BGP speakers that support this specification can
   establish the peering even when presented with unrecognized
   capabilities, so long as all capabilities required to support the
   peering are supported.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Overview of Operations

   When a BGP speaker [RFC4271] that supports capabilities advertisement
   sends an OPEN message to its BGP peer, the message MAY include an
   Optional Parameter, called Capabilities.  The parameter lists the
   capabilities supported by the speaker.

   A BGP speaker determines the capabilities supported by its peer by
   examining the list of capabilities present in the Capabilities
   Optional Parameter carried by the OPEN message that the speaker
   receives from the peer.

   A BGP speaker that supports a particular capability may use this
   capability with its peer after the speaker determines (as described
   above) that the peer supports this capability.  Simply put, a given
   capability can be used on a peering if that capability has been
   advertised by both peers.  If either peer has not advertised it, the
   capability cannot be used.

   A BGP speaker determines that its peer doesn't support capabilities

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   advertisement, if in response to an OPEN message that carries the
   Capabilities Optional Parameter, the speaker receives a NOTIFICATION
   message with the Error Subcode set to Unsupported Optional Parameter.
   (This is a consequence of the base BGP-4 specification [RFC4271] and
   not a new requirement.)  In this case the speaker SHOULD attempt to
   re-establish a BGP connection with the peer without sending to the
   peer the Capabilities Optional Parameter.

   If a BGP speaker that supports a certain capability determines that
   its peer doesn't support this capability, the speaker MAY send a
   NOTIFICATION message to the peer and terminate peering (see Section
   "Extensions to Error Handling" for more details).  An example of when
   this would be done is if the BGP speaker requires that the capability
   in question be used on a peering, for instance if a BGP speaker is
   attempting to establish a peering to exchange IPv6 routes but
   determines that its peer does not support Multiprotocol Extensions
   for BGP-4 [RFC4760].  The Error Subcode in the NOTIFICATION message
   is set to Unsupported Capability.  The message MUST contain the
   capability (capabilities) that causes the speaker to send the
   message.  The decision to send the message and terminate the peering
   is local to the speaker.  If terminated, such peering SHOULD NOT be
   re-established automatically.

   If a BGP speaker receives from its peer a capability which it does
   not itself support or recognize, it MUST ignore that capability.  In
   particular, the Unsupported Capability NOTIFICATION message MUST NOT
   be generated in response to reception of a capability which is not
   supported by the local speaker.

4.  Capabilities Optional Parameter (Parameter Type 2):

   This is an Optional Parameter that is used by a BGP speaker to convey
   to its BGP peer the list of capabilities supported by the speaker.
   The encoding of BGP Optional Parameters is specified in Section 4.2
   of [RFC4271].  The parameter type of the Capabilities Optional
   Parameter is 2.

   The parameter contains one or more triples <Capability Code,
   Capability Length, Capability Value>, where each triple is encoded as
   shown below:

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          | Capability Code (1 octet)    |
          | Capability Length (1 octet)  |
          | Capability Value (variable)  |
          ~                              ~

   The use and meaning of these fields are as follows:

      Capability Code:

         Capability Code is a one octet unsigned binary integer that
         unambiguously identifies individual capabilities.

      Capability Length:

         Capability Length is a one octet unsigned binary integer that
         contains the length of the Capability Value field in octets.

      Capability Value:

         Capability Value is a variable length field that is interpreted
         according to the value of the Capability Code field.

   BGP speakers SHOULD NOT include more than one instance of a
   capability with the same Capability Code, Capability Length, and
   Capability Value.  Note however, that processing of multiple
   instances of such capability does not require special handling, as
   additional instances do not change the meaning of the announced
   capability, thus a BGP speaker MUST be prepared to accept such
   multiple instances.

   BGP speakers MAY include more than one instance of a capability (as
   identified by the Capability Code) with non-zero Capability Length
   field, but with different Capability Value, and either the same or
   different Capability Length.  Processing of these capability
   instances is specific to the Capability Code and MUST be described in
   the document introducing the new capability.

   The Capabilities Optional Parameter (OPEN Optional Parameter Type 2)
   SHOULD only be included in the OPEN message once.  If the BGP speaker
   wishes to include multiple capabilities in the OPEN message, it
   SHOULD do so as discussed above, by listing all those capabilities as
   TLVs within a single Capabilities Optional Parameter.  However, for
   backward compatibility a BGP speaker MUST be prepared to receive an
   OPEN message which contains multiple Capabilities Optional

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   Parameters, each of which contains one or more capabilities TLVs.
   The set of capabilities should be processed in the same way in either
   case, whether it is enumerated within a single Capabilities Optional
   Parameter of the OPEN message, or split across multiple.

5.  Extensions to Error Handling

   This document defines a new Error Subcode, Unsupported Capability.
   The value of this Subcode is 7.  The Data field in the NOTIFICATION
   message MUST list the set of capabilities that cause the speaker to
   send the message.  Each such capability is encoded in the same way as
   it would be encoded in the OPEN message.

   As explained in the Overview of Operations section, the Unsupported
   Capability NOTIFICATION is a way for a BGP speaker to complain that
   its peer does not support a required capability, without which the
   peering cannot proceed.  It MUST NOT be used when a BGP speaker
   receives a capability which it does not understand; such capabilities
   MUST be ignored.

6.  IANA Considerations

   This document defines a Capability Optional Parameter along with a
   Capability Code field.  IANA maintains the registry for Capability
   Code values.  Capability Code value 0 is reserved.  Capability Code
   values 1 through 63 are to be assigned by IANA using the "IETF
   Consensus" policy defined in [RFC5226].  Capability Code values 64
   through 127 are to be assigned by IANA, using the "First Come First
   Served" policy defined in [RFC5226].  Capability Code values 128
   through 255 are for "Private Use" as defined in [RFC5226].

7.  Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing BGP [RFC4272].

8.  Acknowledgements

   The authors would like to thank members of the IDR Working Group and
   the IESG and its Directorates for their review and comments.

9.  References

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9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

9.2.  Informative References

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, January 2006.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              January 2007.

Appendix A.  Comparison with RFC 2842

   In addition to several minor editorial changes, RFC 3392 also
   clarified how to handle multiple instances of the same capability.

Appendix B.  Comparison with RFC 3392

   This document makes minor editorial changes and updated references,
   clarifies the use of the Unsupported Optional Parameter NOTIFICATION
   message, clarifies behavior when the Capabilities parameter is
   included in the OPEN message multiple times, and clarifies
   requirements by changing a number of SHOULDs to MUSTs.

Authors' Addresses

   John G. Scudder
   Juniper Networks


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   Ravi Chandra
   Sonoa Systems


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